Yay, I get to branch out a bit into Pacific history!
The short answer to your question is "colonial borders", but the longer answer is as follows.
Guam and the Marianas are part of Micronesia (the region, not the country), which also includes the Marshall Islands, the Caroline Islands, the Palau Islands, and Kiribati. These areas had a long history of interaction with and colonization by Spain, with Magellan passing through in 1521. The islands gained strategic importance to Spain after the start of its conquest of the Philippines in the 1560s and the establishment of the Manilla Galleon route connecting those territories with New Spain (Mexico), which technically governed them until Mexican independence.
Anyway Spanish contact with the Micronesian islands was off and on, and the Spanish attempted sustained contact and colonial rule in the Marianas in the late 17th century, with the Carolines following later and the Marshalls being formally claimed in the 1870s. These islands were administered as part of the Spanish East Indies along with the Philippines.
Fast forward to the Spanish American War of 1898, when the US destroyed the Spanish fleet at Manila. The US Navy decided to capture Guam to use it effectively as a stop-off point for troops being dispatched from San Francisco to the Philippines via Honolulu (the force left San Francisco on May 25, and Hawaii was formally annexed on June 15). On June 20 the USS Charleston was sent ahead to take possession of Guam - the Spanish authorities on the island didn't even know they were at war with the US, and initially greeted the ship - and it was captured bloodlessly. A small garrison was left on the island and the fleet continued to its destination of the Philippines.
The 1898 Treaty of Paris would see the United States gain the Philippines and Guam from Spain as US colonies. The loss of the Philippines (which had been the center of Spanish administration for the Spanish East Indies), plus the destruction of the Manila fleet meant that the purpose of keeping the rest of the Micronesian islands was questionable for Spain, and the islands were sold to Germany in 1899 for about 17 million marks (Germany had begun to acquire colonies in New Guinea in the 1880s and governed the territories from there). So by 1899 Guam was an American colony, and the rest of the Marianas a German one.
The German Pacific Islands again played a strategic value in war in 1914 - Yap Island was a hub of German undersea cables, and the German East Asia squadron located itself initially among these islands (visiting Pagan in mid-August). The Fleet sailed eastwards (attacking French-controlled Papeete and heading to South America), and the German islands subsequently had little in the way of defenses - they were occupied by Japanese forces in September and October 1914, with Japan announcing its intention to keep the islands after the war (they were seen by the Japanese navy as strategically useful outposts). Japan was confirmed in its possession of these islands, including the Marianas, in the Paris Peace Conference, when they were granted as a League of Nations Mandate territory under Japanese governance.
During the Japanese mandate period, a fair number of Japanese and Korean settlers moved to the mandate territory, especially to the Northern Marianas, where the population on Saipan became overwhelmingly Japanese by the 1930s. The terms of the Mandate required the islands to not be fortified, but when Japan withdrew from the League of Nations in 1933 it both kept the islands and began to develop them (especially the Mariana islands of Saipan, Tinian and Rota) as significant military installations. It uses these installations to attack and capture Guam from the United States at the outbreak of war in December 1941.
The US would go on to recapture Guam and capture the northern Marianas in 1944 as its "leapfrogging" strategy across the Pacific put those islands in range. Major battles were fought on Tinian, Saipan and Guam from June to August 1944, resulting in the deaths of tens of thousands of Japanese troops and also Japanese civilians on Saipan and Tinian, many of the latter from mass suicides. Airfields on Guam and Saipan were of major strategic use for the US from this point, as B-29s based there could strike the Philippines and especially Japan (the B-29s that dropped atomic bombs on Hiroshima and Nagasaki flew there from Tinian).
After the war, the US regained possession of Guam of course, but it received the Northern Marianas, Palau Islands, Carolines and Marshalls as a United Nations Trust Territory, something of an updated version of the League of Nations mandate (ie, the United States was to administer the islands but with an idea of their eventual independence). The a number of referenda were held in the Northern Marians over the idea of merging the Northern Marianas with Guam - inhabitants of the Marianas were consistently in favor, especially because it would give them US citizenship. The United Nations opposed these measures because it wanted the fate of the Trust Territory of the Pacific dealt with as a whole, rather than just in part. Although the Guam legislature had passed motions in favor of unification, especially as part of a step towards eventual US statehood, a 1969 vote saw Guamanians opposed to unification - the Northern Marianas were economically more depressed than Guam, as US military installations in the Marianas had closed, while those in Guam continued in operation, and many Guamanians feared the expense of developing the other islands and/or competing with workers coming to Guam from those islands.
The Northern Marianas were in a bit of a shock at the Guamanian rebuff, and then undertook negotiations with the Nixon Administration over their final status (the Department of Defense wanted a big chunk of Tinian for military use), either as an independent country or as a US commonwealth (a US territory). The Northern Marianas ultimately approved a decision to become a US Commonwealth in 1975, and the islands were officially transferred to US sovereignty on 1986, making the Northern Marianas similar to Puerto Rico in their relation to the United States. Incidentally the Marshall Islands and Caroline Islands (as the Federation of Micronesia) opted for independence and saw this official status gained also in 1986, with Palau finally becoming independent and ending the Trusteeship in 1994. These three countries have Compacts of Free Association with the United States - the US gets military use of their territory (to the exclusion of other countries) and in return gives access to US domestic programs to these countries' citizens (they can live and work in the US, for example).
Thus despite their commonalities and they're both being US commonwealths, Guam and the Northern Marianas went through different colonial historic paths to end up where they are today.
